Nine people are now under investigation in connection with last month's boiler tower collapse at a thermal plant in Ulsan, which killed seven people and injured two.

The Ulsan Metropolitan Police said on Monday that it was looking into three employees of the Korea East-West Power Company, which handled the plant's decommissioning, four workers from contractor HJ Shipbuilding and Construction and two others tied to a subcontractor.

The nine people stand accused of occupational and gross negligence, causing death and injury.

On November 6, a 63-meter tower collapsed during demolition operations, trapping and killing seven out of nine subcontracted workers underneath the rubble.

Police are investigating whether the suspects violated the technical specifications for the decommissioning work, which stipulate that pillars and steel frames must be dismantled from the top level.

The investigators measured the tower pillars at the collapse site and began examining samples of the detached parts.

The Ulsan police said they may expand their investigation to additional suspects after determining the cause of the disaster.